Organic electroluminescence has been studied extensively because of its possible applications in discrete light emitting devices, arrays and displays. Organic materials investigated so far can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The extremely high degree of freedom in organic synthesis promises even more exciting materials in the near future.
Organic electroluminescence at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al., Journal Chem. Phys., Vol. 38,1963, pp. 2024, and in "Recombination Radiation in Anthracene Crystals", Helfrich et al., Physical Review Letters, Vol. 14, No. 7,1965, pp. 229-231. Recent developments have been spurred largely by two reports of high efficiency organic electroluminescence. These are C. W. Tang et al., "Organic electroluminescent diodes", Applied Physics Letters, Vol. 51, No. 12,1987, pp. 913-915, and Burroughs et al., Nature, Vol. 347, 1990, pp. 539. Each utilized transparent ITO (Indium-Tin-Oxide) and metal/glass as substrate. Tang et al. made two-layer organic light emitting devices using vacuum deposited monomeric compounds while Burroughs used spin coated poly(p-phenylenevinylene) (PPV), a polymer.
The advances described by Tang and Burroughs were achieved mainly due to improvements in the design of electroluminescent devices through the selection of appropriate organic multilayers and contact metals. Tang showed that a two-layer structure offers great advantage because, in general, an organic material does not conduct both electrons and holes equally well. A given organic material is usually best suited for only light emission with high efficiency, or for charge transport of one polarity, or for efficient charge injection from a metal contact into the particular organic material. This trend is demonstrated in "Electroluminescence in Organic Films with Three-Layer Structure", C. Adachi et al., Japanese Journal of Applied Physics, Vol. 27, No. 2, 1988, pp. L269-L271 and in "Organic Electroluminescence Devices with a Three-Layer Structure", C. Adachi et al., Japanese Journal of Applied Physics, Vol. 27, No. 4, pp. L713-L715. In these reports, Adachi introduced three-layer structures which separate electron conduction, hole conduction and emission, such that each organic material performed only a single function.
Organic electroluminescent light emitting devices (OLEDs) function much like inorganic light emitting diodes (LED). The simplest possible structure, schematically illustrated in FIG. 1A, consists of an organic emission layer 10 sandwiched between two electrodes 11 and 12 which inject electrons (e-) and holes (h+), respectively. Such a structure has been described in the above mentioned paper of Burroughs et al., for example. The electrons and holes meet in the organic layer 10 and recombine producing light. Improved performance can be achieved when the electrode materials are chosen to match the electron and hole bands of the organic material forming the organic layer 10. Such an improved structure is shown in FIG. 1B. By choosing the right electrode materials 13 and 14, the energy barriers to injection of carriers are reduced, as illustrated. Still, such simple structures perform poorly because nothing stops electrons from traversing the organic layer 10 and reaching the anode 14, or vice versa. FIG. 2A illustrates a device with a large electron barrier 16 such that few electrons are injected, leaving the holes no option but to recombine in the cathode 15. A second problem, shown in FIG. 2B, is that the mobilities of electrons and holes in most known organic materials, especially the conductive ones, differ strongly. FIG. 2B illustrates an example where the holes injected from the anode 18 quickly traverse through the organic layer 19, while the injected electrons move much slower, resulting in recombination near the cathode 17. If the electron mobility in the organic layer 19 were larger than the holes', recombination would occur near the anode 18. Recombination near a metal contact is strongly quenched by the contact which limits the efficiency of such flawed devices.
Tang, as shown in FIG. 3A, separated electron and hole transport functions between two materials mainly to overcome the problems described above, introducing an electron transport layer 20 (ETL) and a hole transport layer (HTL) 21. In "Electroluminescence of doped organic thin films", C. W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989, pp. 3610-3616, it is described that higher carrier mobility was achieved in the two-layer design, which led to reduced device series resistance enabling equal light output at lower operating voltage. The contact metals 22, 23 could be chosen individually to match to the ETL 20 and HTL 21 bands, respectively, while recombination occurred at the interface 24 between the organic layers 20 from either electrode 22, 23. The structure of FIG. 3A blocks electrons from entering the HTL 21 and vice versa by a clever choice of HTL and ETL materials. This feature eliminates non-radiative recombination at the metal contacts as described in FIG. 1A and also promotes a high density of electrons and holes in the same volume leading to enhanced radiative recombination. It was also first disclosed by Tang et al. in this paper that the organic emission layer can be doped with another organic compound, in this case courmarin 540 dye or DCM compounds, to improve the OLED efficiency or alter its emission spectrum. Tang et al. showed that with the proper selection of the organic dopant and host material, recombination is controlled by the dopant.
The structure of preference now has three organic layers, as for example described in "Electroluminescence in Organic Films with Three-Layer Structure", C. Adachi et al., Japanese Journal of Applied Physics, Vol. 27, No. 2, 1988, pp. L269-L271, and in "Electroluminescent Mechanism of Organic Thin Film Devices", C. Adachi et al., 5th International Workshop on Electroluminescence, Helsinki 1990, ACTA Polytechnica Scandinavica, Applied Physics Series No. 170, pp. 215-218. Such a emission layer 30 sandwiched between an ETL 31 and HTL 32, as shown in FIG. 3B. The three-layer structure is partly motivated by the work of F.F. So et al. who showed in their article "Evidence for Excitation Confinement in Crystalline Organic Multiple Quantum Wells", Physical Review Letters, Vol. 66, No. 20, May 20 1991, pp. 2649-2652, that the quantum-well concept is just as valid for organic electroluminescent materials as inorganic semiconductors. The three-layer structure benefits from the confinement of both electrons and holes in the active layer where they can recombine most efficiently. A further advantage is increased specialization of the layers. In two-layer structures, one material must perform both transport and emission necessitating a compromise, whereas the three-layer structure permits the transport and active layers to be chosen exclusively on their conduction and emissive properties, respectively.
Organic LEDs have great potential to outperform conventional inorganic LEDs in many applications. One important advantage of OLEDs and devices based thereon is the price since they can be deposited on large glass substrates, or a wide range of other inexpensive transparent, semitransparent or even opaque substrates at low temperature, rather than on crystalline substrates of limited area at high temperature as is the case for inorganic LEDs. The substrates may even be flexible enabling flexible OLEDs and new types of displays. To date, the performance of OLEDs and devices based thereon is vastly inferior to inorganic ones for several reasons:
High operating voltage: Organic devices require more voltage to inject and transport the charge to the active region (emission layer) which in turn lowers the power efficiency of such devices. High voltage results from the need for high electric fields to inject carriers over energy barriers at the metal/organic interfaces, and from the low mobility of the carriers in the organic transport layers (ETL and HTL) which leads to a large ohmic voltage drop and power dissipation. PA1 Low brightness: Today's OLEDs can produce nearly as many photons per electron as inorganic LEDs, i.e. their quantum efficiency is good. OLEDs lag inorganic LEDs in brightness for the simple reason that comparatively little charge can be conducted through the resistive transport layers (HTL or ETL). This well known effect is referred to as Space Charge Limited Current. Simply put, due to the low mobility of carriers in organic materials, a traffic jam develops which restricts the flux of electrons and holes reaching the emission layer. Better emitter materials cannot offer greatly improved brightness until high conductance transport layers are available. PA1 Reliability: Organic LEDs degrade in air and during operation. Several problems are known to contribute. PA1 Charge injection now can occur between the inorganic contact material and the inorganic component(s) of the organic/inorganic alloy transport layer. The problem of injecting the charge thus is well understood (e.g. metal/semiconductor), and is easily optimized with chemically stable materials using conventional approaches as is elaborated later. PA1 The transfer of carriers from the electrode to the emission region (active region) is improved because of either the high mobility or high carrier concentration of the inorganic components in the organic matrix. PA1 Charge transfer no longer occurs at the electrode/transport layer interface(s), rather within the device. This is highly desirable for two reasons. 1) The alloy microstructure, consisting of dispersed inorganic component(s) within the organic matrix dictates that the inorganic regions will have sharper average feature sizes. At a given operating voltage, sharper features in the conductive inorganic compound(s) correlate directly with increased local electric fields at the inorganic/organic interface resulting in more efficient charge transfer. 2) The dispersed nature of the alloy also produces a drastically higher total contact area between the inorganic and organic compounds. At a given electric field or voltage, the total current is directly proportional to the contact area participating in injection. The advantages are significant when compared to the conventional approach in which the organic/inorganic junction is an extremely smooth interface between two flat layer surfaces.
A) Efficient low field electron injection requires low work function cathode metals like Mg, Ca, F, Li etc. which are all highly reactive in oxygen and water. Ambient gases, and gases coming out of the organic materials during ohmic heating degrade the contacts.
B) Conventional AgMg and ITO contacts still have a significant barrier to carrier injection in known ETL and HTL materials. Therefore, a high electric field is needed to produce significant injection current. This stress from the high field and ohmic heating at the resistive interface contribute to device degradation.
C) The high resistance of conventional carrier transport layers heats the device under operation.
D) Thermal stability of most OLED materials is poor making them sensitive to heating. Upon heating, many amorphous organic materials crystallize into grains. The crystallites have less volume and pack less uniformly than the amorphous solid. The resulting gaps and odd shapes of the crystallites make conduction from one crystallite to the next difficult, increasing resistance and heating in a positive feedback loop, while opening further channels for gaseous contaminants to penetrate, or for neighboring materials to diffuse.
Even the three-layer organic structure (see for example FIG. 3B), requires the transport layers to perform two tasks, charge injection and carrier transport, although they may only be optimized for one task. A natural improvement is a five-layer structure in which two injection layers are added. These new layers would be useful if they split the energy offset between the contact metals and the transport layers. For electrons and holes it is much easier to overcome two small barriers than a single barrier whose height is the sum of the two smaller barriers. Part of an OLED device, in which two smaller barriers are provided, is illustrated in FIG. 4. This device comprises a cathode metal 41 followed by an electron injection layer 40 and an electron transport layer (ETL) 42. The work functions of these three layers 40-42 are chosen such that two small energy steps for electrons replace the larger one which would exist between the cathode and ETL alone. The same applies to holes, too. This enables the injection layer to be chosen for its work function and the transport layer for its mobility.
OLEDs are mainly limited by their contacts and transport layers, not their emission layers. It is thus highly desirable to replace the low work function metals with stable materials that can easily inject charge into the OLED.
While organic materials are highly efficient emissive materials, they are very poor electrical conductors, having both poor mobility and low carrier concentrations. An example of this is Alq3 (tris(8-hydroxyquinoline aluminum)) the most popular electron transport material for OLEDs today. Kepler et al. in "Electron and hole mobility in tris(8-hydroxyquinoline aluminum)" Applied Physics Letters, Vol. 66, No. 26, 1995, pp. 3618-3620, measured the mobilities of electrons and holes in Alq3, obtaining values of 1.4.times.10.sup.-6 and 2.times.10.sup.-8 cm.sup.2 /Vs, respectively. In comparison, amorphous Silicon (Si) deposited on glass, such as is commonly used in solar cells and thin film transistors, has mobilities six orders of magnitude higher, i.e. .about.1cm.sup.2 /Vs. Inorganic metals also have low mobilities, but this is compensated by the huge carrier concentrations which participate in charge conduction. E.g., metals have typical resistivities on the order of 10.sup.-6 Ohm cm or below. Doped semiconductors can be resistivities as low as 10.sup.-3 to 10.sup.-4 Ohm cm. On the other hand, organic conductors (which could otherwise be referred to as insulators), have typical resistivities on the order of 10.sup.6 Ohm cm or more. It therefore makes sense to let each material do what it does best. Polymeric and monomeric materials have different properties which reflect in the performance of OLEDs made from each. This makes devices using both attractive. For example, C. C. Wu et al. reported in their article "Poly(phenylene vinylene)/tris(8-hydroxy) quinoline aluminum heterostructure light emitting diode", Applied Physics Letters, Vol. 66, No. 6, 1995, pp. 653-655, that they used both organic polymer and monomeric layers to form a device. Polymers, having greater molecular weights, do not crystallize like common HTL materials. The disadvantage of this approach is that two different fabrication techniques are required. The delay, transfer etc. introduces contaminants at the interface. J. Kido et al. doped a polymer with monomers, as described in "Organic electroluminescent devices based on molecularly doped polymers", Applied Physics Letters, Vol. 61, No. 7, 1992, pp. 761-763. These approaches do not solve the problems with OLEDs identified above.
S. Tokito et al. described in the article "Organic electroluminescent devices fabricated using a diamine doped MgF.sub.2 thin film as a hole-transporting layer", Applied Physics Letters, Vol. 66, No. 6, 1995, pp. 673-675 a device having an inorganic layer doped with organic components. Their motivation was to avoid recrystallization of the diamine. Their inorganic material, MgF.sub.2, is an insulator, such that all of the hole conductivity was contributed by the organic component introduced.
Other groups have reported inorganic/organic hybrid devices realized by integrating distinct organic layers with inorganic layers. Examples are M. Era et al., "Organic-Inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 Pbl.sub.4 ", Applied Physics Letters, Vol. 65, No. 6, 1994, pp. 676-478. The device of Era et al. used an inorganic perovskite as both the HTL and emission layer, together with an organic oxidiazole ETL. Fujita et al. used n-type epitaxial ZnSe/GaAs as both the ETL and emission layer, together with an organic diamine HTL. The device of Era et al. had poor efficiency because of the poor optical quality of the perovskite active layer. It also suffers from many of the disadvantages of conventional OLEDs in that the ETL conductivity is poor and an unstable cathode metal is needed. The approach of Fujita et al. has none of the desirable properties of OLEDs with respect to conventional inorganic LEDs. The epitaxial ZnSe/GaAs approach is difficult and expensive and limited to small areas by the GaAs substrate. Both the anode and the GaAs substrate strongly absorb the blue light emitted by the ZnSe reducing the light output. Furthermore, the diamine HTL conducts holes much more poorly than many p-type inorganic semiconductors.
As seen from the above, OLEDs, to function best, must solve two problems: efficient recombination and efficient charge transfer from the anode/cathode into the emissive organic material. An ETL and HTL are needed to remove the active region from the quenching contacts for efficient recombination. Low work function cathode metals are the method of choice for efficient charge transfer, since all known electron conducting organics also have low work functions.
However, each of these solutions inhibits performance and degrades device reliability. The price of removing the active layer from the metal contacts for better recombination efficiency are ohmic voltage drops across the HTL/ETL, leading to heating and power consumption. Low work function metals are unstable and unreliable.
As can be seen from the above examples and the description of the state of the art there are two main problems which must be solved to realize OLEDs and displays based thereon with acceptable characteristics.
It is an object of the present invention to provide new and improved organic electroluminescent devices, arrays and displays based thereon.
It is a further purpose of the present invention to provide new and improved organic electroluminescent devices, arrays and displays based thereon with improved efficiency, lower operating voltage, higher brightness and increased reliability.
It is a further object to provide a method for making the present new and improved organic electroluminescent devices, arrays and displays.